1. FIELD OF THE INVENTION
This invention relates to magnetic memory cells having a nonmagnetic, resistive layer which overlies a three layer magnetic "sandwich" structure comprised of two magnetoresistive layers separated by an exchange coupling barrier layer, and wherein a conductive layer overlies the resistive layer at the junction of two of the magnetic cells.
2. RELATED ART
Certain magnetic memories utilize thin films of ferromagnetic, magnetoresistive materials as key elements.
Ferromagnetic materials are materials possessing permanent magnetic dipoles which exhibit a high degree of alignment at room temperature. The net magnetic moment M (or magnetization) of a ferromagnetic material is a measure of the alignment of the dipole moments in the material.
In forming ferromagnetic thin films, the orientation of M can be selected by exposing a ferromagnetic material to a unidirectional external magnetic field during deposition or annealing. The resulting uniaxial anisotropic magnetic film has what is referred to as an easy magnetic axis (aligned with the direction of the externally applied magnetic field), and a hard magnetic axis which is perpendicular to the easy axis.
Further, the magnetic thin film is magneto-resistive. That is, the electrical resistance of the film depends upon the orientation of the easy axis relative to the direction of current flow. The maximum resistance occurs when the magnetization vector and the current direction are parallel, and the minimum resistance occurs when they are perpendicular.
In magnetic memories, data storage lines are formed from the above described magnetoresistive thin films. Data is stored in binary fashion by utilizing a magnetic thin film deposited, as distinct cells, along a sense or bit line. The easy axis is often oriented along the bit line. If the magnetization of a cell is in a first direction along the bit line, the bit is defined as a 1; if the magnetization is in a second direction opposite the first direction, the bit is defined as a 0.
A conductive current strap, or word line, is typically disposed orthogonal to and overlying the bit line. The word line is electrically isolated from the bit line.
The data condition of a cell is sensed or read by passing a sense current through the bit line and a word current through the word line. The sense current is typically very small, e.g. only a few milliamps. The magnetic field associated with the sense current interacts with and rotates the magnetization of the thin film within the plane of the thin film to an oblique position with respect to the easy axis. The rotated magnetization vector will be in a different position for a 0 than for a 1. The magnetic field associated with the word current will either increase or decrease the angle of rotation of the magnetization with respect to the easy axis, depending on the logic state of the bit being sensed.
A sense amplifier, connected across the bit line and responsive to the sense current, will detect a different electrical signal for a 0 than for a 1.
A write operation is similar to a read operation except that the magnitude of the sense and word currents are increased so that together the magnetic fields associated with the sense and word currents are sufficient to flip the rotated magnetization vector from one logic state to the other. Some magnetic memories form the easy axis perpendicular (or transverse) to the longitudinal axis of the bit line. Read and write operations in that case are similar to those of the longitudinal cell.
The magnetization of the magnetic cells disposed along a bit line must be independent of the magnetization of adjacent cells. This is achieved by cutting the magnetic material at bit cell junctions or reducing the width of the bit line at cell junctions to an extent sufficient to decouple the magnetization of adjacent cells.
The sense current in the bit line must flow through cell junctions without undue constraint, so conductive paths or "shorting bars" of a good electrical conductor are formed between the cells and, in the case of the narrowed bit line, over the narrowed junction portion.
The process for manufacturing the magneto-resistive bit lines involves depositing one or more layers of material over an underlying layer or surface, and etching the deposited material to a desired configuration. Several problems with the memory result from the manufacturing process.
First, in forming the shorting bars, typically a via is cut through an SiO.sub.2 layer to expose the bit cell junction. Once exposed to deposition chamber atmosphere, a thin oxide or other high resistance layer will form on the upper surface of the bit cell junction (or the memory cell will be exposed to other contaminants). Although this oxide and/or contaminant is sputtered off prior to shorting bar deposition, a less than perfect interface is formed. If the shorting bar is at the end of a bit cell line, it will often be configured as an electrical contact to a bit line sense amplifier. The interface resistance will substantially increase the contact resistance of this electrical contact, further reducing the already small signal current.
The cell junction at the end of a bit cell line could be specially processed, but this will add complicated processing steps and increase production costs.
Second, each via that is cut in the oxide adds another potential process error site to the magnetic memory. Thus minimizing the number of vias needed to produce the bit cell lines is advantageous.
Third, a thin resistive layer may be formed over the magnetic thin film to protect the thin film during processing. One material useful as the resistive layer, nitrogen doped tantalum (TaN.sub.x), has an etch rate similar to SiO.sub.2. If the etch of the SiO.sub.2 is not carefully controlled, the resistive layer may be cut into deeply or even cut through. A further etch barrier to protect the magnetic thin film at sites where multiple etches will occur, such as bit cell junctions, is highly desirable.
Finally, if the shorting bars can be made coincident with the magnetic elements, the minimum "pitch" (i.e. the center to center spacing of the magnetic elements) can be decreased because the spacing requirements of via and metal are no longer a consideration.